The present invention is in the field of photochromic glass and specifically relates to photochromic glass compositions which are both chemically strengthenable and suitable for use in the manufacture of photochromic sheet glass by direct drawing from a melt.
The basis patent in the field of photochromic glass, U.S. Pat. No. 3,208,860 to Armistead and Stookey, discloses a broad range of silicate glass compositions exhibiting reversible phototropic properties. These glasses are rendered reversibly phototropic through the inclusion of specified quantities of silver halide in the glass composition, and through appropriate heat treatment of the glass after forming to cause the precipitation and growth of silver halide crystallites in the glass. These crystallites are small enough to be invisible, yet are darkenable under the action of light to reduce the light transmitting capabilities of the glass. Upon shielding from light, the crystallites fade to the colorless state, restoring the original light transmission characteristics of the glass. In photochromic glasses these darkening and fading cycles may be repeated indefinitely without fatigue.
A major use for photochromic glasses exhibiting reversible darkening under the action of visible light has been in the manufacture of photochromic ophthalmic lenses. U.S. Pat. No. 3,197,296 to Eppler and Stookey, for example, describes a family of refractive index-corrected silicate glasses comprising silver halides which are useful for prescription lenses. These glasses exhibit, in conventional 2 mm thicknesses, photochromic properties sufficiently developed for prescription ophthalmic applications, and have refractive index characteristics compatible with conventional lens grinding procedures.
Ophthalmic lens manufacture normally comprises the pressing of glass lens blanks of optical quality from a melt followed by grinding and polishing of the lens blanks to specified prescriptions. It will be appreciated that the manufacture of non-prescription photochromic glass lenses, e.g., sunglass lenses, in large quantity by processes requiring grinding and polishing is not only expensive but also wasteful of material. For these reasons it would be desirable to provide less costly means for producing photochromic glass sheet for lenses or other applications. It would be particularly desirable to provide means for producing such sheet in large quantities and in a surface and bulk quality sufficient for optical use. The sheet could then be inexpensively sagged to curvatures required for lenses, windshields, or other sheet glass configurations.
Minimum requirements of photochromic glass sheet for applications such as described would be high optical quality, good chemical durability, and high strength and good photochromic darkenability even in moderate thickness. If the sheet is to be suitable for lightweight sunglass lenses, it must also be chemically strengthenable in order to meet FDA requirements for eyeglass lens safety. Federal safety requirements cannot be routinely met in lightweight glass sheet of moderate thickness (1.3-1.7 millimeters) in the absence of tempering, or by utilizing air tempering processes.
One method of improving the darkenability of photochromic glasses for thin sheet applications is to increase the silver halide content thereof in the manner described by Stookey in U.S. Pat. No. 3,449,103. However, this expedient requires that the glass be rapidly quenched from the melt in order to avoid haze in the glass due to macroscopic silver halide crystal formation. Pressing and rolling are processes which provide relatively rapid quenching of the glass below silver halide crystallization temperatures; however, these processes do not normally provide glass articles or glass sheet of optical surface quality because of surface marking by the mold or roller surfaces utilized to quench the glass.
Drawing processes are known which comprise direct formation of glass sheet from a melt, and in most of these processes the glass sheet surfaces are not contacted by mold or roller surfaces until after the glass has cooled sufficiently to resist surface marking. The widely known sheet draw processes include the Colburn process, the Fourcault process, and the Pittsburgh Plate or Pennvernon process. These processes utilize rollers to draw the sheet up from a glass melt; but can provide glass of near-optical quality and without surface marks in thicknesses on the order of about 1.5 millimeters. The downdraw sheet-forming processes described in U.S. Pat. Nos. 3,338,696 and 3,682,609 to Dockerty are particularly suitable for providing thin, lightweight sheet of controlled uniform thickness and optical quality at relatively low viscosities. German Patentschift No. 2,125,232 describes attempts to provide photochromic glass of conventional composition utilizing forming methods of this type.
Unfortunately, these known sheet drawing processes do not provide the rapid quenching action of conventional pressing procedures, presenting an uncertainty regarding the feasibility of producing haze-free highly-darkenable silver halide photochromic glass therefrom.
Moreover, all of these processes involve holding substantial volumes of glass at rather low temperatures to obtain useful sheet forming viscosities in the 10.sup.4 -10.sup.6 poise range. These volumes of glass are also required to be in prolonged contact with the refractory metals or ceramics utilized as the means for forming the drawn sheet. Such processes impose severe constraints on glass composition because of the formidable liquidus and glass stability problems associated with the handling and processing of glass at relatively low temperatures and high viscosities.
These problems are further aggravated by the fact that the attainment of the required degree of strengthenability in silicate glasses of the normal type require the inclusion in the glass of some minimum quantities of exchangeable alkali ions. In the case of the photochromic glasses herein described, the presence of lithium oxide is required. Unfortunately, the presence of lithium has a detrimental effect on low temperature glass stability, particularly in the presence of refractory ceramic surfaces.
Therefore, in order to be useful for the manufacture of lightweight photochromic glass sheet of optical quality utilizing conventional drawing processes, a glass composition must exhibit specified chemical and physical properties in addition to the aforementioned properties of optical clarity, chemical durability, chemical strengthenability and good photochromic darkening in low or moderate thicknesses. In particular, the glass must exhibit high viscosity at the liquidus and excellent stability against devitrification at forming viscosities despite prolonged contact with refractory metal and ceramic materials presently available for use as forming means in sheet draw processes.
In terms of specific constraints relating to glass properties for these processes, a glass composition must exhibit a viscosity at its liquidus temperature of at least about 10.sup.4 and preferably about 10.sup.5 poises, and it must exhibit excellent long term stability against devitrification and interfacial crystallization in contact with refractory metals and ceramics such as platinum, mullite, sillimanite, and high density alumina-containing refractories used to contain or form molten glass. This stability must be maintained down to temperatures in the range corresponding to glass viscosities of 10.sup.4 -10.sup.6 poises, which are the viscosities at which the glass is normally formed.
In addition, the glass provided must be one which is chemically strengthenable to an unabraded modulus of rupture strength of at least 45,000 psi and a depth of compression layer of at least 3.5 mils utilizing known ion-exchange strengthening processes. This combination of properties is required to enable the glass to routinely meet Federal safety requirements. Finally, the photochromic properties of the glass must be such that the glass exhibits, in thickness not exceeding about 1.7 millimeters, a luminous transmittance in the darkened or activated state (darkened luminous transmittance) of not more than about 25%, and a fading rate such that the glass exhibits a faded luminous transmittance at least 1.5 times the darkened luminous transmittance in a five-minute fading interval.
In addition, it is required for most sunglass applications that these darkening and fading characteristics be obtained in glasses exhibiting a luminous transmittance in the clear or unactivated state (clear luminous transmittance) of at least about 60%, typically in the range of about 60-92%.